Abstract This chapter presents the new functionalities of Cormas, a generic agent- based modeling (ABM) platform dedicated to common-pool resource management. As free software, Cormas is used by an international community of researchers willing to understand the relationships between societies and their environment. It is intended to facilitate the design ofABMas well as the monitoring and analysis of simulation scenarios. Cormas has taken an innovative direction oriented toward the collective design of models and interactive simulation. In accordance with the principles of participatory methods and serious games, many experiments devel- oped with Cormas combine two layers of complexity: the natural dynamic of the system, simulated by the computer, and the stakeholders’ interactions and decisions played by the actors. Between these two extremes, a range of intermediate situa- tions exists where some decisions are human and others are computer specified. The term hybrid agent simulation covers these intermediary situations. The main idea is to enable the stakeholders to interact with the execution of a simulation by modi- fying the behavior of the agents and the way they use the resources. Thus, it is possible to collectively explore scenarios to better understand how a desired situation may be reached. This may feed back into the collective design of the model. As our intention is to involve more deeply the stakeholders into the modeling process, it is necessary to have an easily changeable tool to act on the simulation and to modify the conceptual model on the fly. After having explained the purpose and the philosophy of the companion modeling, this chatper presents how the Cormas functionalities (asymmetry of information, agent manipulation, modification of behavior, stepping back, and distributed simulation) are put into practice through three experiments with stakeholders facing actual environmental challenges.